The invention relates to optical communication devices and systems and, more particularly, to optical threshold devices.
In the field of optical communication, there is a need for fast threshold devices. Such devices may be used for enhancing Signal to Noise Ratio (SNR), regenerating signals, discriminating between signals, and multiplexing/demultiplexing pulses of different magnitudes.
U.S. Pat. No. 5,917,979 (xe2x80x9cthe ""979 patentxe2x80x9d) describes an optical threshold device having a non-linear loop mirror design including an ultra-fast Non Linear Element (NLE), such as a Solid-state (semiconductor) Optical Amplifier (SOA). The ""979 patent demonstrates the potential efficiency of using a NLE, such as a SOA, as an ultra-fast device, which may be capable of producing desired phase shifts as a function of pulse intensity for interference purposes. However, the device described in the ""979 patent suffers from various imperfections and limitations.
The device of the ""979 patent relies on an asymmetric coupler as the input and output terminals to the loop mirror for producing pulses of different directions and intensities propagating along the loop. In the range of high input intensities, a relatively large ratio between the high and low peaks of the pulse intensity is essential for producing output signals, which may be modulated based on the different phase shifts produced for different intensities. To produce a high intensity ratio, the input coupler should be significantly asymmetric. However, an asymmetric coupler at the loop input cannot null the output signals for low intensity level input signals, e.g., for signals below a certain intensity threshold, by recombining the pulses returning to the coupler after completing their travel around the loop. Therefore, it is impossible to maintain a high amplitude ratio between the pulses that propagate along the loop, in different directions, while maintaining good threshold performances, e.g., zero output signals for the range of low input intensities. Accordingly, the design of the device the ""979 patent is based on a tradeoff, i.e., optimization, between a high amplitude ratio (i.e., a high splitting ratio of the coupler) and the ability to null the output signals for the range of low level input signals. This leads to a compromise between the performances of the threshold device in the ranges of low and high level signals at the device input.
In a Variety of applications, threshold discrimination is used for discriminating only between two different intensity levels. Still, for such applications and others, it is important that the output of the threshold device would be substantially zero at low input intensities and non-zero for high input intensities, e.g., intensities above a certain threshold level.
It is an object of embodiments of an aspect of the present invention to provide an improved optical threshold device. Exemplary embodiments of the invention may utilize principles of asymmetric nonlinear Mach Zehnder Interferometer (MZI) threshold devices, non-linear optical loop mirror threshold devices, and variations of asymmetric nonlinear MZI devices and optical loop mirror devices. The threshold devices in accordance with exemplary embodiments of the invention may produce output signal intensities approaching zero in response to low level input intensities and significantly non-zero output signals for higher input intensities.
In accordance with embodiments of an aspect of the invention there is provided an optical threshold device including an optical loop having first and second terminals and including at least one non-linear optical element and at least one optical attenuator; and an optical coupler able to couple a first portion of an input signal to the first terminal and a second portion of the input signal to the second terminal, wherein the optical loop is able to produce a first return signal at the second terminal and a second return signal at the first terminal, and wherein the optical coupler is able to combine the first and second return signals into an output signal.
In accordance with embodiments of another aspect of the invention there is provided an optical threshold device including an optical loop having first and second terminals, an optical coupler associated with the first and second terminals and configured to couple a first portion of an input signal to the first terminal and a second portion of the input signal to the second terminal, at least one non-linear optical element and at least one optical attenuator on the optical loop configured to shift the phase of the first portion by a first phase shift and to shift the phase of the second portion by a second phase shift different from the first phase shift, wherein the optical coupler is able to combine a first return signal at the second terminal and a second return signal at the first terminal into an output signal responsive to the difference between the first and second phase shifts.
In accordance with embodiments of yet another aspect of the invention there is provided an optical threshold device including an optical loop having fist and second terminals, an optical coupler associated with the first and second terminals and configured to couple a first portion of an input signal to the first terminal and a second portion of the input signal to the second terminal at least one non-linear optical element on the optical loop able to receive and transmit the first portion in a first propagation direction and to receive and transmit the second portion in a second propagation direction, and at least one optical attenuator on the optical loop able to attenuate the first portion before it is received by the non-linear element and to attenuate the second portion after it is transmitted through the non-linear element, wherein the non-linear optical element is further able to produce a phase difference between the first and second portions responsive to a difference in amplitude between the first and second portions before the portions are transmitted through the non-linear element, and wherein the optical coupler is able to combine the first and second portions after the portions are transmitted through the non-linear element into an output signal responsive to the phase difference.
In accordance with embodiments of a further aspect of the invention there is provided an optical threshold device including an input, an output, an optical coupler associated with the input and the output and able to couple an input signal from the input into a first input portion and a second input portion, and an optical loop having first and second terminals associated with the optical coupler to receive the first and second input portions, respectively, and including at least one non-linear optical element and at least one optical attenuator, the optical loop able to produces a first return signal at the second terminal and a second return signal at the first terminal, wherein the optical coupler is farther able to combine the first and second return signals into an output signal at the output by interference between the first and second return signals.
In accordance with embodiments of an additional aspect of the invention there is provided an optical threshold method including receiving an input signal, coupling a first portion of the input signal to a first terminal of an optical loop including a non-linear optical element and an optical attenuator, coupling a second portion of the input signal to a second terminal of the optical loop, producing first and second return signals at the second and first terminals, respectively, by simultaneously propagating the first and second input portions, respectively, through the loop in opposite directions, and combining the first and second return signals into an output signal.